fix(sxbh-ui-p0): a11y + self-host hero (P0 x5)

P0-1 define --teal-600,--amber-600 (were 0-def); P0-2 global :focus-visible (was 0 hits); P0-3 contrast 18 pairs recolored all >=4.5(text)/3.0(ui); P0-4 de-pexels hero -> /images/hero-home.webp (webp 84.9KB); P0-5 scope .fade-in under html.js + inline js class. Whitelist: style.css,index.html,images/hero-home.webp.

index.html

@@ -1,4 +1,5 @@
 <html lang="zh-CN"><head>
+<script>document.documentElement.className+=' js';</script>
 <meta charset="UTF-8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>陕西博海网络科技：AI与CMA驱动的企业数字化转型服务商</title>
@@ -481,7 +482,7 @@
 <!-- CONTACT -->
 <section class="contact-section" id="contact">
   <div class="section-inner">
-    <div class="section-label fade-in" style="color:rgba(255,255,255,.4);">
+    <div class="section-label fade-in" style="color:rgba(255,255,255,.72);">
       <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round">
         <path d="M4 4h16c1.1 0 2 .9 2 2v12c0 1.1-.9 2-2 2H4c-1.1 0-2-.9-2-2V6c0-1.1.9-2 2-2z"></path>
         <polyline points="22,6 12,13 2,6"></polyline>